Like a moth that can't get too far from the flame, Mercury is almost
always lost in the solar glare. But once in a while, this tiny world,
not much bigger than our Moon, sparkles in the morning or evening
twilight glow.

This planet moves the fastest of all nine planets in the Solar System.
It is named for the fastest of the ancient Roman gods, the messenger of
the gods, Mercury (in Greek mythology, Hermes). See All Things Mercury'sHermes description.

We here at Hermograph Press have been observing Mercury for over 5
decades. It doesn't get the PR that most of the other planets get--lost in
their glow, as it were. So starting here we are going to take you on a
journey, a series of Mercurian orbits, each one passing through five aspects of this planet. At
the end of each Web page, click on the NEXT ORBIT link to continue on your way, until you return back here. Enjoy the trip!

Note: These pages are under continual refinement.

The Basic Facts

Size

4879 km, or
about 3050 miles in diameter. This makes Mercury about 38% the diameter
of Earth, about 40% bigger than our moon, and the second smallest
planet in the solar system, if you still want to include Pluto (we don't...).

The iron core is quite large in relation to the whole planet--1800-1900
km in radius, leaving a 500-600 km silicate crust (the equivalent area
of earth, the mantle and surface lithosphere, is much greater in
percentage of the earth's radius than the core). This makes Mercury's
core the second densest in the solar system, slightly behind Earth's.

Gravity

Also 38%
of Earth values. You'd weigh 38 pounds for every 100 pounds of Earth
weight. By coincidence, this is the same weight you'd have on Mars, a
larger planet. The reason is Mercury is denser--has more stuff per
cubic inch on the average--than Mars. The more stuff (in science terms,
more mass), the more gravitational pull there is, and therefore the more
you will weigh, all other things being equal. As written above, the density of
Mercury is almost the same as Earth's, 5.5 grams/cubic centimeter. This
signifies it probably also has an iron core, like Earth.

Escape Velocity

You'd have to move at the speed of 4.2 kilometers per second to escape
Mercury's grasp. On Earth you'd have to get moving at a speed of 11.2
km/s.

Albedo

Although it gets more and brighter sunlight than any other planet, it reflects a
mere 11% of the sunlight that hits its surface. This makes Mercury a dark world, the darkest major body in the solar system.

Rotation (Day) and Revolution (Year)

It's
a LONG workday, from an Earth perspective. It takes 58.6 Earth days
for Mercury to spin once on its axis. This is 2/3rds of its orbital
period (year). The planet spins counterclockwise (like the Earth) and
also orbits counterclockwise. HOWEVER, it would be about 176 Earth
days between sunrises! Why?

Suppose you start out with the sun in view in front of you from some
point of Mercury's orbit. It would be noon Mercury time. We notice
that it had been lined up with a certain star in the background. Very
very slowly, because of Mercury's rotation, we'd see the sun move
across the Mercurian sky, just as the Earth spin makes the sun and stars
seem to move across our sky. The orbital motion (revolution) also
changes from where we view the Sun and stars, both on Mercury and
Earth.. So as the Sun appears to move from East to West in our daytime
sky, it also appears to move through the star fields as well. On
Mercury, we'd see the Sun gradually separating from the star it was
lined up with when we started, as it moves across the Mercurian sky. On
Earth our planetary revolution makes the Sun move slowly through the
Zodiac constellations through one Earth year.

When one complete spin (day) of the planet has occurred, 58 Earth days
later, the star is once again at the noon position. But 58 days later
Mercury is at a different part of its orbit; it has only completed
2/3rds of an orbit. We have to look in a DIFFERENT direction to see the
Sun, off to one side of where it was before. It will be almost 120
more days before IT is back at the noon position, too. [This is the very
same phenomenon that makes the Moon take ~30 days to repeat its phases
but only 27 days to return to the same
constellation/star position in the sky, the fact that as the moon
revolves around us, we are also changing our viewpoint by revolving
around the Sun. An analogy
to this using clock hands is found in Issue 1 of The Classroom Astronomer.

This makes for an interesting kind of sky overhead on Mercury,
especially if you were at one of the two special "hot spots" on Mercury,
Caloris Basin. Because Mercury's orbit is not circular, the size of
the sun will change dramatically over the "day." The Sun would rise
small and grow in size as it moves to the overhead position.
Furthermore, because Mercury moves faster when closer to the Sun than
when it is farthest away, the apparent motion of the Sun in the sky
caused by the orbital motion is briefly faster than the shifting caused
by the planet's spinning. At the time of closest approach to the Sun,
the differences in speed cause the Sun to temporarily move
BACKWARDS(!) in the sky. Then it resumes its normal westward motion,
begins to shrink in size by sunset, 88 Earth days after Sunrise. Then,
for 88 days, it is night.

On Earth, things are not so dramatic. We don't spin one and a half
times per year, we spin just over 365 times per year. The difference
between two successive noon appearances of the Sun is but 23 hours and
56 minutes, not days!

How many Mercury days and years old are you?

By the stars, a Mercury day is 58.6 Earth days or 0.16 Earth years. A
Mercury year is 0.24 years. For each full Earth year you are alive,
you are 6.25 Mercury-days-old, and 4.17 Mercury-years-old. A forty-four
Earth-years-old on his birthday would be 275 Mercury-days-old and a whopping
183.33 Mercury-years-old!

Orbital Characteristics

On average it moves at 29 miles per second,
fastest planet in the solar system.

Mercury
has the second-most non-circular, elliptical orbit of all the original
nine planets.
Only tinier Pluto's orbit is less circular. On the mean, Mercury
is 0.387 Astronomical Units away from the Sun (1 A.U. is the Earth's
average orbital distance, just under 93,000,000 miles or 149.6 million
kilometers). This means it is about 36 million miles
from the Sun. Light, the fastest moving object in the universe, takes
about three minutes to reach Mercury after it leaves the Sun, 8.3
minutes to reach Earth. And speaking of the Sun, it would appear from 2
to more than three times as large in Mercury's skies than in Earth's.
Why? Because of that elliptical orbit, which has an eccentricity
(non-circularity factor) of 0.206;
Mercury can be as close to the Sun as about 28 million miles, and as far
away as about 43 million miles.

This also changes the amount of
sunlight that hits the little world. At its farthest orbital position
Mercury gets 4X as much sunlight as Earth. At its nearest point, it
gets 11X! Visiting Mercury? Bring your sunblock!!

Compared to the Earth's orbit, Mercury's orbit is tipped about 7
degrees. Unlike the Earth, which has its axis tipped 23 degrees with
respect to its orbital plane, Mercury is not tipped at all with respect
to its own orbit. It therefore has no seasons.

For the technical among you, the longitude of the ascending node (the
direction in space, seen from the Sun, where Mercury goes from below the
ecliptic plane of Earth's orbit to above it) is 48o. The longitude of perihelion (the point in space as seen from the Sun
where Mercury is closest to the sun) is 77o. But these do drift in time.

Surface Conditions

It's HOT! No, It's COLD! No, It's BOTH!

On the day side, temperatures
peak at 800 oF (425oC) and drop to -275oF at night (-170oC).
Because of Mercury's eccentric orbit and the resonance between
rotation and revolution, the places around longitudes 0 and 180 degrees
get 2.5X as much radiation as places near longitudes 90 and 270 degrees.

Yet, near Mercury's poles appear be small deposits of ice, in craters
that are
in perpetual shadow. These were detected by radar (1991,
using the Arecibo antenna, and the Goldstone radio telescope along with
the Very Large Array in New Mexico) and have not been confirmed by any
visual sightings. Recently, ice was detected by the Messenger mission
in orbit around the planet. (We'll come back to this point in a future
ORBIT.) The ice present is due to being perpetually in crater
rim shadows and may have come from either volcanic outgassing or
bombardment by comets.

Atmosphere

Though
Mercury's gravity is too weak to hold onto an atmosphere; nevertheless
there is a trace of one. There appears to be an extremely miniscule
amount of Helium (42% of the total Mercurian atmosphere), an equally
miniscule amount of Sodium (some researchers dispute the presence of
Sodium at all), 15% Oxygen, with 1% of the total Neon and Argon, a grand
total atmospheric pressure of only 1/150th that of Earth (about 10-15bar--1
bar is Earth normal air pressure). It is not enough to act as a
blanket, transferring daytime heat to the night side as happens on Earth.
It is not enough to stop even the smallest meteoroid from hitting the
surface and making a crater. It is enough, apparently, though, to cause
very very weak and dim auroral glows, detectable only by sensitive
instruments on-board the Mariner 10 spacecraft. The atmosphere is
believed to be temporarily trapped particles of the solar wind.

Miscellaneous

Mercury
has no known moon. It does have a very weak magnetic field, but strong
enough to deflect the sun's solar wind with a bow shock (like that of
Earth, or of a boat moving through water). The field is only
1/30th that of Earth's.

Logo

The
symbol of the planet, used by astrologers and astronomers alike, is
described as based either on Mercury the god's wand or cadeuceus around
which are two snakes, or Mercury in his winged hat.